1. Field of the Invention
The present invention relates to imaging devices for converting optical signals to electric signals, and more particularly, to an x-ray CT (computerized tomography) imaging device system for medical applications.
2. Related Art
Recently, x-ray imaging device systems have been proposed as a possible application of TFT (thin-film transistor) arrays (for example, see U.S. Pat. No. 4,689,487). X-ray CT systems are also considered as a possible application of TFT arrays.
As shown in FIG. 8, an x-ray CT system has a structure that an x-ray source 301 and a detector 302 are located on opposite sides relative to an object person 303. The detector 302 is composed of a scintillation detector for converting an x-ray to an optical signal, and a photodiode for converting an optical signal to an electric signal. An x-ray emitted from the x-ray source 301 passes through the object person 303 to reach the detector 302. The detector 302 outputs an electric signal corresponding to the intensity of the x-ray, thereby forming an image.
In a helical scan type x-ray CT system, the x-ray source 301 and the detector 302 helically rotates in the direction of the body axis of the object person 303, and thereby sequentially collects x-ray images. While the detector 302 collects about 2,000 x-ray images per second, the x-ray source 301 keeps emitting x-ray. A sectional view of the object person 303 is formed by combining the x-ray images collected by the detector 302.
In recent x-ray CT systems, multi-slice type detectors are employed in order to shorten the imaging time. FIG. 9 shows a multi-slice type detector 302. A detecting element 304 provided to each horizontally extending line collects one slice of x-ray image. In FIG. 9, eight detecting elements 304 are provided so as to collect eight slices of an image at a time. Since the imaging time can be shortened, the time during which the object-person must be still can be shortened, thereby decreasing the total x-ray dose.
A detector using a TFT array is proposed to replace the detectors now on the market. With such a detector, it is possible to easily increase the number of slices, thereby easily shortening the imaging time and decreasing the x-ray dose. Further, it is possible to reduce the pixel size, thereby obtaining high-definition images.
In order to obtain a high-definition image, it is important to employ an x-ray-to-charge conversion method, which is called the direct conversion method, in addition to reducing pixel size. Unlike the indirect conversion method, in which an x-ray is temporarily converted to a visible light, with the direct conversion method, an x-ray is directly converted to a charge. Since a high electric field is applied to a photoelectric conversion film, leakage of charge to adjacent pixels can be avoided, so that the pixel size determines the resolution level.
Since a high voltage of a few kV is supplied to the photoelectric conversion film in the direct conversion method, it is necessary to take a countermeasure against the dielectric breakdown of the TFT array.
Japanese Patent Laid-Open Publication No. 2000-58804 discloses an example of the technique for preventing dielectric breakdown. FIG. 6 shows this technique, in which a negative power supply 240 is connected to one end of a photoelectric conversion film 203, and an accumulative pixel capacitance 202 is connected to the other end thereof. Electrons are accumulated in the accumulative pixel capacitance 202 when an x-ray is received. Further, a source of a TFT 201 is connected to the other end of the photoelectric conversion film 203, and a gate thereof is supplied with a negative voltage. Accordingly, under normal conditions, the gate-source voltage Vgs is a negative voltage, and the TFT 201 is in the off state. When an x-ray is received and electrons are accumulated in the accumulative pixel capacitance 202, the gate-source voltage Vgs is decreased. When a certain level of signal is received to accumulate electrons sufficiently enough to make the gate-source voltage Vgs in the vicinity of zero, the TFT 201 is turned on. Then, a reset switch 253 allows the excessive electrons to be discharged through the signal line and an integral capacitance 252. In this way, voltage applied to the accumulative pixel capacitance 202 is restricted to under a predetermined level, and it is possible to prevent the dielectric breakdown of insulating layer.
Although the dielectric breakdown preventing technique disclosed in Japanese Patent Laid-Open Publication No. 2000-58804 is effective when the x-ray is emitted in a pulse irradiation manner, if the x-ray is emitted in a continuous irradiation manner, noise is increased, resulting in a deterioration of the quality of images. This will be described below.
FIGS. 7A and 7B show the x-ray irradiation period and the signal readout period in the pulse irradiation mode, and the continuous irradiation mode, respectively. FIG. 7A is the timing chart of the pulse irradiation, and FIG. 7B is the timing chart of the continuous irradiation.
In the pulse irradiation mode, the x-ray irradiation period and the signal readout period are temporally separated, as shown in FIG.7A. In this case, when an excessive dose of x-ray is received, signals beyond the predetermined level (undesired signal) flow through the signal line toward the detection amplifier 251 side. However, since the undesired signal flows only during the x-ray irradiation period, which is temporally different from the signal readout period, the detection amplifier 251 is in the reset state. Accordingly, no undesired signal is detected by the detection amplifier 251.
In the continuous irradiation mode, however, the x-ray irradiation and the signal readout are simultaneously performed, as shown in FIG. 7B. Accordingly, undesired signals are always produced and flow toward the detection amplifier 251. Therefore, the detection amplifier 251 simultaneously detects a pixel signal and an undesired signal, and the undesired signal is recognized as a noise, thereby deteriorating the quality of image.
FIG. 10 schematically shows an x-ray imaging device system using TFTs. An x-ray emitted from an x-ray source 51 passes through an object person 52 to reach a TFT imaging device 53, which converts the x-ray to a corresponding analog electric signal. The converted analog signals are converted to digital signals in a time sequential manner by an A/D converter 57, and stored in an image memory 58. The image memory 58 is capable of storing one or more images. A control section 63 transmits a control signal to the image memory 58 so that the image memory 58 sequentially stores data items to predetermined addresses. A processing section 59 retrieves a data item from the image memory 58, processes it, and outputs the result to the image memory 58. The processed data item from the image memory 58 is converted to an analog signal by a D/A converter 60, and displayed on a monitor 61 as an x-ray image. For example, x-ray moving images can be obtained by capturing images at 30 frames per second. However, in such an imaging mode for displaying moving images, if x-rays are continuously emitted to display moving images, noise due to undesired signals is possibly caused as in the case of the x-ray CT system.